The present invention relates to a process for preparing pre-expanded particles of a polyolefin resin. More particularly, the invention relates to a process for preparing pre-expanded polyolefin resin particles which are applicable to a process for producing foams by heating pre-expanded particles with a heating medium such as steam in a mold which is able to be closed but unable to be sealed, that is, in a mold which is able to be closed but is permeable to a fluid heating medium.
As a process for pre-expanding particles of a polyolefin resin, there are, for example, a process disclosed in DE-OS 2,107,683 and a process disclosed in Japanese Examined Patent Publication No. 56-1344.
According to the above processes, in a pressure vessel, particles of the polyolefin resin are dispersed in water, impregnated with a volatile blowing agent and heated to a temperature close to the melting point of the resin particles. Since the polyolefin resin pre-expanded particles are easily fused together or blocking (agglomeration) of the particles easily occurs in the processes, as the blowing agent used, a blowing agent having a relatively small plasticizing property to the polyolefin resin, typically dichlorodifluoromethane (hereinafter referred to as "R-12"), is used. However, when using a blowing agent having a small plasticizing property to the resin, the obtained pre-expanded particles are small in cell diameter, for example, when using R-12, the cell diameter is generally not more than 200 .mu.m, and it is difficult to control the cell diameter to a desired value. Moreover, the use of R-12 is not preferable because R-12 destroys the ozone layer encompassing the earth.
Japanese Unexamined Patent Publication No. 58-199125). discloses decreasing the diameter of cells by using an inorganic fine powder such as talc or silica, but no technique for increasing the diameter of cells has been disclosed.
When the cell diameter of the pre-expanded particles is too small, it is necessary, prior to molding the pre-expanded particles, to further impart expandability to the pre-expanded particles (Japanese Unexamined Patent Publication No. 59-187086) or to conduct aging under pressure (Japanese Examined Patent Publication No. 60-10017). However, the adoption of such steps is economically disadvantegeous because the required steps are increased, and even if conducting the steps, the allowable range of molding conditions is narrow.
Japanese Examined Patent Publication No. 56-1344 discloses in Example 2 that an industrial butane is used as the blowing agent. However, when using industrial butane as the blowing agent, while the cell diameter of the pre-expanded particles can be made large, it is unavoidable that the pre-expanded particles are fused together or blocking of the pre-expanded particles occurs. Furthermore, since the industrial butane escapes from the obtained pre-expanded particles, the pressure inside the cells decreases during replacement of the intracellular butane gas with air, thus resulting in large shrinkage of the pre-expanded particles.
It is an object of the present invention to eluminate the above-mentioned defects concerning the pre-expanded particles of the polyolefin resin.
Namely, an object of the present invention is to provide a process for preparing polyolefin resin pre-expanded particles which are not fused together or are not agglomerated to each other to form a block, and are large in cell diameter and exhibit less shrinkage.
These and other objects of the present invention will become apparent from the description hereinafter.